Turn on -Wunused and fix the mistakes it uncovers.

Change-Id: I023d2d8b547fbc21d4124bb7510d42b06a0dc501
diff --git a/tests/Android.mk b/tests/Android.mk
index db8dd8c..811c12a 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -34,7 +34,7 @@
 test_cflags = \
     -fstack-protector-all \
     -g \
-    -Wall -Wextra \
+    -Wall -Wextra -Wunused \
     -Werror \
     -fno-builtin \